Certain rotorcraft are capable of taking off, hovering and landing vertically. One such rotorcraft is a helicopter, which has one or more main rotors that provide lift and thrust to the aircraft. The main rotors not only enable hovering and vertical takeoff and landing, but also enable forward, backward and lateral flight. These attributes make helicopters highly versatile for use in congested, isolated or remote areas. Another such rotorcraft is a tiltrotor aircraft, which has a set of proprotors that can change their plane of rotation based on the operation being performed. Tiltrotor aircraft typically have a pair of nacelles mounted near the outboard ends of a fixed wing with each nacelle housing a propulsion system that provides torque and rotational energy to a proprotor. The nacelles are rotatable relative to the fixed wing such that the proprotors have a generally horizontal plane of rotation providing vertical thrust for takeoff, hovering and landing, much like a conventional helicopter, and a generally vertical plane of rotation providing forward thrust for cruising in forward flight with the fixed wing providing lift, much like a conventional propeller driven airplane.
It has been found, however, that hot and/or high environments present numerous challenges for rotorcraft. For example, as the altitude and temperature increase, rotorcraft have reduced range, reduced payload capacity, reduced maneuverability and reduced station time. In addition, operations at high density altitudes have increased power requirements and increased fuel burn. One solution for increasing the altitude density ceiling of rotorcraft is the use of larger, more powerful engines. This solution, however, results in a permanent weight penalty and a decrease in rotorcraft efficiency. Accordingly, a need has arisen for rotorcraft having an increased altitude density ceiling without the weight penalty or decreased efficiency of using larger engines.